adduser,addgroup: introduce and use CONFIG_LAST_ID

Changes adduser.c, addgroup.c and Config.src to set and use CONFIG_LAST_ID.

function                                             old     new   delta
adduser_main                                         841     865     +24
addgroup_main                                        407     425     +18

Signed-off-by: Tito Ragusa <farmatito@tiscali.it>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>